In secondary oil recovery systems, oil is recovered by pumping water into the ground through a secondary hole near the oil deposit. The water seeps through the ground, and as it is under pressure, the water forces the oil to the surface through a primary drill hole. This oil flow, however, will contain oil, water and solid particles. Most of the oil is separated from the rest of the flow at the surface, and the residual liquid is recirculated back into the ground through the secondary drill hole. The use of this residual water substantially reduces the amount of fresh or sea water which must be continually added while the system is in operation.
The principal drawback of this method is that the solid particles will be carried by the residual water back into the secondary drill hole, and they may be large enough to block the seepage holes in the ground thereby preventing the water from reaching the oil deposit. Due to the very small size of the particles which would cause such a blockage, conventional filtering has been unable to correct the problem. An alternative solution has been to shut down the system until the water can be satisfactorily cleaned. However, conventional particle detection methods are not useful in determining when to shut down, as the recirculated flow also contains oil droplets, which would be simultaneously detected. While the droplets do not otherwise adversely affect the system, the prior art detectors cannot discriminate between the solid particles and the droplets, and the presence of the droplets confuses and makes meaningless any readings that are made.
Further, the recirculated flow may also contain large oil slugs, the presence of which indicates that the oil-water separator devices are not functioning properly. Accordingly, it is desirable to detect these slugs as well.